Bone cement and methods of use thereof

ABSTRACT

A bone cement comprising an acrylic polymer mixture which is formulated to have a relatively high viscosity for a relatively long window, due to distributions of molecular weights and/or sizes of acrylic beads.

RELATED APPLICATIONS

The present application claims the benefit under 119(e) of 60/825,609filed Sep. 14, 2006, the disclosure of which is incorporated herein byreference.

The present application is related to U.S. patent application Ser. No.11/461,072 filed on Jul. 31, 2006 and entitled “Bone Cement and Methodsof Use Thereof”, which is a Continuation-in-Part of U.S. applicationSer. No. 11/360,251 filed on Feb. 22, 2006, entitled “Methods, Materialsand Apparatus for Treating Bone and Other Tissue” and is also aContinuation-in Part of PCT/IL2005/000812 filed on Jul. 31, 2005. Thedisclosures of these applications are incorporated herein by reference.

The present application is related to PCT application PCT/IL2006/052612filed on Jul. 31, 2006 and entitled “Bone Cement and Methods of Usethereof” the disclosure of which is incorporated herein by reference.

The present application is related to Israel application No. 174347filed on Mar. 16, 2006 and entitled “Bone Cement and Methods of Usethereof” the disclosure of which is incorporated herein by reference.

The present application is also related to a series of US provisionalapplications entitled “Methods, Materials and Apparatus for TreatingBone and Other Tissue”: 60/765,484 filed on Feb. 2, 2006; 60/762,789filed on Jan. 26, 2006; 60/738,556 filed Nov. 22, 2005; 60/729,505 filedOct. 25, 2005; 60/720,725 filed on Sep. 28, 2005 and 60/721,094 filed onSep. 28, 2005. The disclosures of these applications are incorporatedherein by reference.

The present application is related to PCT application PCT/IL2006/000239filed on Feb. 22, 2006; U.S. provisional application 60/763,003,entitled “Cannula” filed on Jan. 26, 2006; U.S. provisional applicationNo. 60/654,495 entitled “Materials, devices and methods for treatingbones”. filed Feb. 22, 2005; U.S. Ser. No. 11/194,411 filed Aug. 1,2005; IL 166017 filed Dec. 28, 2004; IL 160987 filed Mar. 21, 2004; U.S.Provisional Application No. 60/654,784 filed on Jan. 31, 2005; U.S.Provisional Application No. 60/592,149 filed on Jul. 30, 2004; PCTApplication No. PCT/IL2004/000527 filed on Jun. 17, 2004, IsraelApplication No. 160987 filed on Mar. 21, 2004, U.S. ProvisionalApplications 60/478,841 filed on Jun. 17, 2003; 60/529,612 filed on Dec.16, 2003; 60/534,377 filed on Jan. 6, 2004 and 60/554,558 filed on Mar.18, 2004; U.S. application Ser. No. 09/890,172 filed on Jul. 25, 2001;U.S. application Ser. No. 09/890,318 filed on Jul. 25, 2001 and U.S.application Ser. No. 10/549,409 entitled “Hydraulic Device for theinjection of Bone Cement in Percutaneous Vertebroplasty filed on Sep.14, 2005. The disclosures of all of these applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to bone cement, formulations thereof andmethods of use thereof.

BACKGROUND OF THE INVENTION

It is common to employ cement to repair bones in a variety of clinicalscenarios.

For example, compression fractures of the vertebrae, which are a commonoccurrence in older persons, cause pain and/or a shortening (or otherdistortion) of stature. In a procedure known as vertebroplasty cement isinjected into a fractured vertebra. Vertebroplasty stabilizes thefracture and reduces pain, although it does not restore the vertebra andperson to their original height. In vertebroplasty the cement istypically injected in a liquid phase so that resistance to injection isnot too great. Liquid cement may unintentionally be injected outside ofthe vertebra and/or may migrate out through cracks in the vertebra.

In another procedure, known as kyphoplasty, the fracture is reduced byexpanding a device, such as a balloon inside the vertebra and theninjecting a fixing material and/or an implant. Kyphoplasty reduces theproblem of cement leakage by permitting a lower pressure to be used forinjection of the cement.

In general, polymeric cements become more viscous as the polymer chaingrows by reacting directly with the double bond of a monomer.Polymerization begins by the “addition mechanism” in which a monomerbecomes unstable by reacting with an initiator, a volatile molecule thatis most commonly a radical (molecules that contain a single unpairedelectron). Radicals bond with monomers, forming monomer radicals thatcan attack the double bond of the next monomer to propagate the polymerchain. Because radicals are so transient, initiators are often added inthe form of an un-reactive peroxide form which is stable in solution.Radicals are formed when heat or light cleaves the peroxide molecule.For applications in which high temperatures are not practical (such asthe use of bone cement in vivo), peroxide is typically cleaved by addinga chemical activator such as N, N-dimethyl-p-toluidine. (Nussbaum D A etal: “The Chemistry of Acrylic Bone Cement and Implication for ClinicalUse in Image-guided Therapy”, J Vase Interv Radiol (2004); 15:121-126;the content of which is fully incorporated herein by reference).

Examples of commercially available viscous bone cements include, but arenot limited to, CMW® Nos. 1, 2⁻ and 3 (DePuy Orthopaedics Inc.; Warsaw,Ind., USA) and Simplex™-P and -RO (Stryker Orthopaedics; Mahwah, N.J.,USA). These cements are characterized by a liquid phase after mixing andprior to achieving a viscosity of 500 Pascal-second. In a typical usescenario, these previously available cements are poured, while in aliquid phase, into a delivery device.

There have also been attempts to reduce cement leakage by injecting moreviscous cement, for example, during the doughing time and the beginningof polymerization. However, the viscous materials, such as hardeningPMMA, typically harden very quickly once they reach a high viscosity.This has generally prevented injection of viscous materials inorthopedic procedures.

Some bone fixing materials, such as polymethylmethacrylate (PMMA), emitheat and possibly toxic materials while setting.

US patents and publication U.S. Pat. Nos. 4,969,888, 5,108,404,6,383,188, 2003/0109883, 2002/0068974, U.S. Pat. Nos. 6,348,055,6,383,190, 4,494,535, 4,653,489 and 4,653,487, the disclosures of whichare incorporated herein by reference describe various tools and methodsfor treating bone.

US patent publication 2004/0260303, the disclosure of which isincorporated herein by reference, teaches an apparatus for deliveringbone cement into a vertebra.

Pascual, B., et al., “New Aspects of the Effect of Size and SizeDistribution on the Setting Parameters and Mechanical Properties ofAcrylic Bone Cements,” Biomaterials, 17(5): 509-516 (1996) considers theeffect of PMMA bead size on setting parameters of cement. This articleis fully incorporated herein by reference.

Hernandez, et al., (2005) “Influence of Powder Particle SizeDistribution on Complex Viscosity and Other Properties of Acrylic BoneCement for Vertebroplasty and Kyphoplasty” Wiley International ScienceD01:10:1002jbm.b.30409 (pages 98-103) considers the effect of PMMA beadsize distribution on setting parameters of cement. Hernandez suggeststhat it is advantageous to formulate cement with a liquid phase tofacilitate injection. This article is fully incorporated herein byreference.

U.S. Pat. No. 5,276,070 to Arroyo discloses use of acrylic polymers witha molecular weight in the range of 0.5 to 1.5 million Daltons informulation of bone cement. The disclosure of this patent is fullyincorporated herein by reference.

U.S. Pat. No. 5,336,699 to Cooke discloses use of acrylic polymers witha molecular weight of about one hundred thousand Daltons in formulationof bone cement. The disclosure of this patent is fully incorporatedherein by reference.

SUMMARY OF THE INVENTION

A broad aspect of the invention relates to a bone cement characterizedby a rapid transition from separate liquid monomer and powdered polymercomponents to a single phase characterized by a high viscosity when thecomponents are mixed together with substantially no intervening liquidphase. Optionally, high viscosity indicates 500 Pascal-second or more.Mixing is deemed complete when 95-100% of the polymer beads are wettedby monomer. In an exemplary embodiment of the invention, mixing iscomplete in within 60, optionally within 45, optionally within 30seconds.

In an exemplary embodiment of the invention, the cement is characterizedby a working window of several minutes during which the viscosityremains high prior to hardening of the cement. Optionally, viscosityduring the working window does not vary to a degree which significantlyinfluences injection parameters. In an exemplary embodiment of theinvention, viscosity increases by less than 10% during a sub-window ofat least 2 minutes during the working window. Optionally, the viscosityin the working window does not exceed 500, optionally 1,000, optionally1,500, optionally 2,000 Pascal-second or lesser or greater orintermediate values. In an exemplary embodiment of the invention, theworking window lasts 6, optionally 8, optionally 10, optionally 15minutes or lesser or greater or intermediate times. Optionally, ambienttemperature influences a duration of the working window. In an exemplaryembodiment of the invention, the cement can be cooled or heated toinfluence a length of the working window.

An aspect of some embodiments of the invention relates to formulationsof bone cement which rely upon two, optionally three or more,sub-populations of polymer beads which are mixed with liquid monomer.

According to exemplary embodiments of the invention, sub-populations maybe characterized by average molecular weight (MW) and/or physical sizeand/or geometry, and/or density. In an exemplary embodiment of theinvention, size based and MW based sub-populations are definedindependently. In an exemplary embodiment of the invention, thesub-populations are selected to produce desired viscositycharacterization and/or polymerization kinetics. Optionally, the polymerbeads comprise polymethylmethacrylate (PMMA) and/or a PMMA styrenecopolymer. Optionally, PMMA is employed in conjunction with amethylmethacrylate (MMA) monomer.

Optionally, a high molecular weight sub-population contributes to arapid transition to a high viscosity with substantially no liquid phase.Optionally, a low molecular weight subpopulation contributes to a longerworking window.

Optionally, a sub-population with small size contributes to rapidwetting of polymer beads with monomer solution. In an exemplaryembodiment of the invention, rapid wetting contributes to a directtransition to a viscous cement with substantially no liquid phase.

In some cases a small percentage of beads may not belong to any relevantsub-population. The small percentage may be, for example 1%, 1.5%, 2%,3%, 4%, 5% or lesser or intermediate or greater percentages.

In one exemplary embodiment of the invention, there are at least twosub-populations of PMMA polymer beads characterized by molecularweights. For example, a first sub-population comprising 95 to 97% (w/w)of the total PMMA beads can be characterized by an average MW of270,000-300,000 Dalton; a second sub-population (2-3% w/w) can becharacterized by an average MW of 3,500,000-4,000,000 Dalton; and athird sub-population (0-3% w/w) can be characterized by an average MW of10,000-15,000 Dalton.

In an exemplary embodiment of the invention, the polymer beads arecharacterized by a high surface area per unit weight. Optionally, thebeads have a surface area of 0.5 to 1, optionally 0.5 to 0.8 optionallyabout 0.66 m²/gram or intermediate or lesser or greater values.Optionally, the high surface area/weight ratio improves wettingproperties and/or shortens polymerization times, for example bycontributing to polymer monomer contact.

In an exemplary embodiment of the invention, a cement characterized byan immediate transition to high viscosity is injected during a workingwindow in a vertebroplasty of kyphoplasty procedure. Optionally,injection is under sufficient pressure to move fractured bone, such asvertebral plates of a collapsed vertebra. Optionally, injection ofviscous cement under high pressure contributes to fracture reductionand/or restoration of vertebral height.

In an exemplary embodiment of the invention, the material (e.g., bonecement) includes processed bone (from human or animals origin) and/orsynthetic bone. Optionally, the cement has osteoconductive and/orosteoinductive behavior. Additional additives as commonly used in bonecement preparation may optionally be added. These additives include, butare not limited to, barium sulfate and benzoyl peroxide.

According to some embodiments of the invention, a working window lengthis determined by an interaction between an immediate effect and a lateeffect. In an exemplary embodiment of the invention, the immediateeffect includes MMA solvation and/or encapsulation of PMMA polymerbeads. The immediate effect contributes to a high viscosity of theinitial mixture resulting from solvation and/or friction between thebeads. The late effect is increasing average polymer MW as the beadsdissolve and the polymerization reaction proceeds. This increasingaverage polymer MW keeps viscosity high throughout the working window.

In an exemplary embodiment of the invention, a set of viscosityparameters are used to adjust a cement formulation to produce a cementcharacterized by a desired working window at a desired viscosity.

In an exemplary embodiment of the invention, there is provided a bonecement comprising an acrylic polymer mixture, the cement characterizedin that it achieves a viscosity of at least 500 Pascal-second within 180seconds following initiation of mixing of a monomer component and apolymer component and characterized by sufficient biocompatibility topermit in-vivo use.

Optionally, the viscosity of the mixture remains between 500 and 2000Pascal-second for a working window of at least 5 minutes after theinitial period.

Optionally, the working window is at least 8 minutes long.

Optionally, the mixture includes PMMA.

Optionally, the mixture includes Barium Sulfate.

Optionally, the PMMA is provided as a PMMA/styrene copolymer.

Optionally, the PMMA is provided as a population of beads divided intoat least two sub-populations, each sub-population characterized by anaverage molecular weight.

Optionally, a largest sub-population of PMMA beads is characterized byan MW of 150,000 Dalton to 300,000 Dalton.

Optionally, a largest sub-population of PMMA beads includes 90-98% (w/w)of the beads.

Optionally, a high molecular weight sub-population of PMMA beads ischaracterized by an average MW of at least 3,000,000 Dalton.

Optionally, a high molecular weight sub-population of PMMA beadsincludes 2 to 3% (w/w) of the beads.

Optionally, a low molecular weight sub-population of PMMA beads ischaracterized by an average MW of less than 15,000 Dalton.

Optionally, a low molecular weight sub-population of PMMA beads includes0.75 to 1.5% (W/W) of the beads.

Optionally, the PMMA is provided as a population of beads divided intoat least two sub-populations, each sub-population characterized by anaverage bead diameter.

Optionally, at least one bead sub-population characterized by an averagediameter is further divided into at least two sub-sub-populations, eachsub-sub-population characterized by an average molecular weight.

Optionally, the PMMA is provided as a population of beads divided intoat least three sub-populations, each sub-population characterized by anaverage bead diameter.

Optionally, the cement further includes processed bone and/or syntheticbone.

Optionally, the cement is characterized in that the cement achieves aviscosity of at least 500 Pascal-second when 100% of a polymer componentis wetted by a monomer component.

Optionally, the viscosity is at least 800 Pascal-second.

Optionally, the viscosity is at least 1500 Pascal-second.

Optionally, the viscosity is achieved within 2 minutes.

Optionally, the viscosity is achieved within 1 minute.

Optionally, the viscosity is achieved within 45 seconds.

In an exemplary embodiment of the invention, there is provided a bonecement comprising:

a polymer component; and

a monomer component,

wherein, contacting the polymer component and the monomer componentproduces a mixture which attains a viscosity greater than 200Pascal-second within 1 minute from onset of mixing and remains below2000 Pascal-second until at least 6 minutes from onset of mixing.

Optionally, the polymer component comprises an acrylic polymer.

In an exemplary embodiment of the invention, there is provided aparticulate mixture formulated for preparation of a bone cement, themixture comprising:

(a) 60 to 80% polymer beads comprising a main sub-populationcharacterized by an MW of 150,000 Dalton to 300,000 Dalton and a highmolecular weight sub-population characterized by an MW of 3,000,000Dalton to 4,000,000 Dalton; and(b) 20 to 40% of a material which is non-transparent with respect toX-ray.

Optionally, the polymer beads comprise a third subpopulationcharacterized by an MW of 10,000 Dalton to 15,000 Dalton.

In an exemplary embodiment of the invention, there is provided a methodof making a polymeric bone cement, the method comprising:

(a) defining a viscosity profile including a rapid transition to aworking window characterized by a high viscosity;(b) selecting a polymer component and a monomer component to produce acement conforming to the viscosity profile; and(c) mixing the polymer component and a monomer component to produce acement which conforms to the viscosity profile.

In an exemplary embodiment of the invention, there is provided a cementkit, comprising:

(a) a liquid component including a monomer, and

(b) a powder component including polymeric beads,

characterized in that said powder component is provided in asubstantially non-normal distribution of at least one of molecularweight of the polymeric beads and size of powder particles such that acement mixed from the kit has both an increased immediate viscosity andan increased working window as compared to a cement having asubstantially normal distribution.

Optionally, the substantially non-normal distribution is a skeweddistribution.

Optionally, the substantially non-normal distribution comprises arelatively small component including higher molecular weight beads.Optionally, said component has an average molecular weight of at least afactor of 2 of an average molecular weight of said polymeric beads.Optionally, said factor is at least 3 or is at least 5.

Optionally, the substantially non-normal distribution comprises arelatively small component including smaller sized particles.

BRIEF DESCRIPTION OF THE FIGURES

Exemplary non-limiting embodiments of the invention will be describedwith reference to the following description of embodiments inconjunction with the figures. Identical structures, elements or partswhich appear in more than one figure are generally labeled with a sameor similar number in all the figures in which they appear, in which:

FIG. 1 is a flow diagram illustrating an exemplary method 100 ofpreparation and behavior of exemplary cements according to the presentinvention;

FIG. 2 is a graph of viscosity profiles depicting viscosity(Pascal-second) as a function of time (minutes) for an exemplary cementaccording to the invention and an exemplary prior art cement;

FIGS. 3 and 4 are graphs indicating viscosity as Newtons of appliedforce per unit displacement (mm) under defined conditions for exemplarycements according to the invention and illustrate the time window forinjection which is both early and long; and

FIG. 5 is a graph showing the results of bead size distributionanalysis, for a bead formulation in accordance with an exemplaryembodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Overview of Preparation ofExemplary Bone Cement

FIG. 1 is a flow diagram illustrating preparation and behavior ofexemplary cements according to some embodiments of the presentinvention.

In an exemplary embodiment of the invention, a liquid monomer and apowdered polymer component of a bone cement are combined 110.Optionally, liquid monomer is poured onto powdered polymer.

According to various embodiments of the invention, average polymermolecular weight and/or polymer molecular weight distribution and/orpolymer bead size is precisely controlled in order to influencepolymerization kinetics and/or cement viscosity. Alternatively oradditionally, polymer and/or monomer components may contain ingredientswhich are not directly involved in the polymerization reaction.

In an exemplary embodiment of the invention, the polymer (e.g. anacrylic polymer such as PMMA) beads are divided into two or moresub-populations. Optionally, the sub-populations are defined bymolecular weight (MW). In an exemplary embodiment of the invention, theaverage molecular weight of the acrylic polymer in all the beads is inthe range of about 300,000 to 400,000, optionally about 373,000 Dalton.This average MW for all beads was determined experimentally for a batchof beads which produced cement with a desired polymerization profile.

Optionally, the polymer beads are provided as part of an acrylic polymermixture, for example a mixture including barium sulfate.

At 112 the components are mixed until the polymer is wetted by themonomer. Optionally, when wetting is 95 to 100% complete, the mixturehas achieved a desired high viscosity, for example 500 Pascal-second ormore. Optionally, mixing 112 is complete within 1, 5, 10, 15, 30, 60,90, 120 or 180 seconds. In a modern medical facility, it can beadvantageous to shorten the mixing time in order to reduce the demand onphysical facilities and/or medical personnel. A savings of even 1 to 2minutes with respect to previously available alternatives can besignificant. In an exemplary embodiment of the invention, mixing 112 isconducted in a mixing apparatus of the type described in co-pendingapplication U.S. Ser. No. 11/428,908, the disclosure of which is fullyincorporate herein by reference.

After mixing 112 is complete, a working window 114 during which thecement remains viscous but has not fully hardened occurs. According tovarious exemplary embodiments of the invention, working window 114 maybe about 2, 5, 8, 10, 15 or 20 minutes or intermediate or greater times.The duration of the working window may vary with the exact cementformulation and/or ambient conditions (e.g. temperature and/orhumidity). Formulation considerations include, but are not limited topolymer MW (average and/or distribution), polymer bead size,concentrations of non-polymerizing ingredient and polymer:monomer ratio.

Working window 114, permits a medical practitioner sufficient time toload a high pressure injection device and inject 120 the cement into adesired location. Optionally, an injection needle or cannula is insertedinto the body prior to, or concurrent with mixing 112 so that window 114need only be long enough for loading and injection 120. Exemplaryinjection systems are disclosed in co-pending application U.S. Ser. No.11/360,251 entitled “Methods, materials, and apparatus for treating boneand other tissue” filed Feb. 22, 2006, the disclosure of which is fullyincorporated herein by reference.

In an exemplary embodiment of the invention, hardening 116 to a hardenedcondition occurs after working window 114. The cement hardens 116 evenif it has not been injected.

Advantages with Respect to Relevant Medical Procedures

In an exemplary embodiment of the invention, cement with a viscosityprofile as' described above is useful in vertebral repair, for examplein vertebroplasty and/or kyphoplasty procedures.

Optionally, use of cement which is viscous at the time of injectionreduces the risk of material leakage and/or infiltrates into theintravertebral cancellous bone (interdigitation) and/or reduces thefracture [see G Baroud et al, Injection biomechanics of bone cementsused in vertebroplasty, Bio-Medical Materials and Engineering 00 (2004)1-18]. Reduced leakage optionally contributes to increased likelihood ofa positive clinical outcome.

In an exemplary embodiment of the invention, the viscosity of the bonecement is 500, optionally 1,000, optionally 1,500, optionally 2,000Pascal-second or lesser or greater or intermediate values at the timeinjection begins, optionally 3, 2 or 1 minutes or lesser or intermediatetimes after mixing 112 begins. Optionally, the viscosity does not exceed2,000 Pascal-second during working window 114. In an exemplaryembodiment of the invention, this viscosity is achieved substantially assoon as 95-100% of the polymer beads are wetted by monomer.

Cement characterized by a high viscosity as described above mayoptionally be manually manipulated.

In an exemplary embodiment of the invention, cement is sufficientlyviscous to move surrounding tissue as it is injected. Optionally, movingof the surrounding tissue contributes to fracture reduction and/orrestoration of vertebral height.

An injected volume of cement may vary, depending upon the type and/ornumber of orthopedic procedures being performed. The volume injected maybe, for example, 2-5 cc for a typical vertebral repair and as high as8-12 cc or higher for repairs of other types of bones. Other volumes maybe appropriate, depending for example, on the volume of space and thedesired effect of the injection. In some cases, a large volume ofviscous cement is loaded into a delivery device and several vertebraeare repaired in a single medical procedure. Optionally, one or morecannulae or needles are employed to perform multiple procedures.

Viscous cements according to exemplary embodiments of the invention maybe delivered at a desired flow rate through standard orthopedic cannulaeby applying sufficient pressure. Exemplary average injection rates maybe in the range of 0.01 to 0.5 ml/sec, optionally about 0.05, about0.075 or 0.1 ml/sec or lesser or intermediate or greater average flowrates. Optionally, the flow rate varies significantly during aninjection period (e.g., pulse injections). Optionally, the flow rate iscontrolled manually or using electronic or mechanical circuitry. In anexemplary embodiment of the invention, medical personnel view the cementas it is being injected (e.g. via fluoroscopy) and adjust a flow rateand/or delivery volume based upon observed results. Optionally, the flowrate is adjusted and/or controlled to allow a medical practitioner toevaluate progress of the procedure based upon medical images (e.g.fluoroscopy) acquired during the procedure. In an exemplary embodimentof the invention, the cement is sufficiently viscous that advances intothe body when pressure is applied above a threshold and ceases toadvance when pressure is reduced below a threshold. Optionally, thethreshold varies with one or more of cement viscosity, cannula diameterand cannula length.

Comparison of Exemplary Formulations According to Some Embodiments ofthe Invention to Previously Available Formulations

Although PMMA has been widely used in preparation of bone cement,previously available PMMA based cements were typically characterized bya persistent liquid state after mixing of components.

In sharp contrast, cements according to some exemplary embodiments ofthe invention are characterized by essentially no liquid state.Optionally, a direct transition from separate polymer and monomercomponents to a highly viscous state results from the presence of two ormore sub-populations of polymer beads.

As a result of formulations based upon bead sub-populations, a viscosityprofile of a cement according to an exemplary embodiment of theinvention is significantly different from a viscosity profile of apreviously available polymer based cement (e.g. PMMA) with a similaraverage molecular.

Because the viscosity profile of previously available PMMA cements istypically characterized by a rapid transition from high viscosity tofully hardened, these cements are typically injected into bone in aliquid phase so that they do not harden during injection.

In sharp contrast, exemplary cements according to the invention remainhighly viscous during a long working window 114 before they harden. Thislong working window permits performance of a medical procedure ofseveral minutes duration and imparts the advantages of the highviscosity material to the procedure.

It should be noted that while specific examples are described, it isoften the case that the formulation will be varied to achieve particulardesired mechanical properties. For example, different diagnoses maysuggest different material viscosities which may, in turn lead toadjustment of one or more of MW (average and/or distribution), bead sizeand bead surface area.

In an exemplary embodiment of the invention, the cement is mixed 112 andreaches high viscosity outside the body. Optionally the materials aremixed under vacuum or ventilated. In this manner, some materials withpotentially hazardous by-products can be safely mixed and then used inthe body.

In an exemplary embodiment of the invention, the cement is formulated sothat its mechanical properties match the bone in which it will beinjected/implanted. In an exemplary embodiment of the invention, thecement is formulated to mechanically match healthy or osteoporotictrabecular (cancellous) bone. Optionally, the mechanical properties ofthe bone are measured during access, for example, based on a resistanceto advance or using sensors provided through a cannula or by takingsamples, or based on x-ray densitometry measurements. In an exemplaryembodiment of the invention, strength of the cement varies as a functionof one or more of a size of the high MW sub-population and/or arelationship between bead size and bead MW.

In general, PMMA is stronger and has a higher Young modulus thantrabecular bone. For example, healthy Trabecular bone can have astrength of between 1.5-8.0 mega Pascal and a Young modulus of 60-500mega Pascal. Cortical bone, for example, has strength values of 65-160mega Pascal and Young modulus of 12-40 giga Pascal. PMMA typically hasvalues about half of Cortical bone (70-120 mega Pascal strength).

FIG. 2 is a plot of viscosity as a function of time for an exemplarybone cement according to the present invention. The figure is not drawnto scale and is provided to illustrate the principles of exemplaryembodiments of the invention. The end of a mixing process is denoted astime 0. Mixing is deemed to end when 95-100% of acrylic polymer beadshave been wetted with monomer. The graph illustrates an exemplary bonecement which enters a high viscosity plastic phase upon mixing so thatit has substantially no liquid phase.

FIG. 2 illustrates that once a high viscosity is achieved, the viscosityremains relatively stable for 2, optionally 5, optionally 8 minutes ormore. In an exemplary embodiment of the invention, this interval ofstable viscosity provides a working window 114 (indicated here as Δt₁)for performance of a medical procedure. In an exemplary embodiment ofthe invention, stable viscosity means that the viscosity of the cementchanges by less than 200 Pascal-second during a window of at least 2minutes optionally at least 4 minutes after mixing is complete.Optionally, the window begins 1, 2, 3, 4 or 5 minutes after mixingbegins or lesser or intermediate times. In an exemplary embodiment ofthe invention, the viscosity of the cement remains below 1500,optionally 2000 Pascal-second for at least 4, optionally at least 6,optionally at least 8, optionally at least 10 minutes or intermediate orgreater times from onset of mixing.

For purposes of comparison, the graph illustrates that an exemplaryprior art cement reaches a viscosity comparable to that achieved by anexemplary cement according to the invention at time zero at a time ofapproximately 10.5 minutes post mixing and is completely set by about15.5 minutes (Δt₂).

A working window 114 during which viscosity is between 400 and 2000Pascal-second for an exemplary cement according to some embodiments ofthe invention (Δt₁) is both longer and earlier than a comparable windowfor an exemplary prior art cement (Δt₂). Optionally, (Δt₁) beginssubstantially as soon as mixing is complete.

Exemplary Cement Formulations

According to various exemplary embodiments of the invention, changes inthe ratios between a powdered polymer component and a liquid monomercomponent can effect the duration of working window 114 and/or aviscosity of the cement during that window. Optionally, these ratios areadjusted to achieve desired results.

In an exemplary embodiment of the invention, the powdered polymercomponent contains PMMA (69.3% w/w); Barium sulfate (30.07% w/w) andBenzoyl peroxide (0.54% w/w).

In an exemplary embodiment of the invention, the liquid monomercomponent contains MMA (98.5% v/v); N, N-dimethyl-p-toluidine (DMPT)(1.5% v/v) and Hydroquinone (20 ppm).

In a first exemplary embodiment of the invention, 20±0.3 grams ofpolymer powder and 9±0.3 grams of liquid monomer are combined (weightratio of ˜2.2:1).

In a second exemplary embodiment of the invention, 20±0.3 grams ofpolymer powder and 8±0.3 grams of liquid are combined (weight ratio of2.5:1).

Under same weight ratio of second exemplary embodiment (2.5:1), a thirdexemplary embodiment may include a combination of 22.5±0.3 grams ofpolymer powder and 9±0.3 grams of liquid.

In general, increasing the weight ratio of polymer to monomer produces acement which reaches a higher viscosity in less time. However, there isa limit beyond which there is not sufficient monomer to wet all of thepolymer beads.

Optionally the powdered polymer component may vary in composition andcontain PMMA (67-77%, optionally 67.5-71.5% w/w); Barium sulfate(25-35%; optionally 28-32% w/w) and Benzoyl peroxide (0.4-0.6% w/w) andstill behave substantially as the powder component recipe set forthabove.

Optionally the liquid monomer component may vary in composition andcontain. Hydroquinone (1-30 ppm; optionally 20-25 ppm) and still behavesubstantially as the liquid component recipe set forth above.

Viscosity Measurements Over Time for Exemplary Cements

In order to evaluate the viscosity profile of different exemplarybatches of cement according to some embodiments of the invention, a bulkof pre-mixed bone cement is placed inside a Stainless Steel injectorbody. Krause et al. described a method for calculating viscosity interms of applied force. (“The viscosity of acrylic bone cements”,Journal of Biomedical Materials Research, (1982): 16:219-243). Thisarticle is fully incorporated herein by reference.

In the experimental apparatus an inner diameter of the injector body isapproximately 18 mm. A distal cylindrical outlet has an inner diameterof approximately 3 mm and a length of more than 4 mm. This configurationsimulates a connection to standard bone cement delivery cannula/boneaccess needle. A piston applies force (F), thus causing the bone cementto flow through the outlet. The piston is set to move with constantvelocity of approximately 3 mm/min. As a result, piston deflection isindicative of elapsed time.

The experimental procedure serves as a kind of capillary extrusionrheometer. The rheometer measures the pressure difference from an end toend of the capillary tube. The device is made of an 18 mm cylindricalreservoir and a piston. The distal end of the reservoir consist of 4 mmlong 3 mm diameter hole. This procedure employs a small diameter needleand high pressure. Assuming steady flow, isothermal conditions andincompressibility of the tested material, the viscous force resistingthe motion of the fluid in the capillary is equal to the applied forceacting on the piston measured by a load cell and friction. Results arepresented as force vs. displacement. As displacement rate was constantand set to 3 mm/min, the shear rate was constant as well. In order tomeasure the time elapses from test beginning, the displacement rate isdivided by 3 (jog speed).

FIG. 3 indicates a viscosity profile of a first exemplary batch ofcement according to the invention as force (Newtons) vs. displacement(mm). The cement used in this experiment included a liquid component anda powder component as described above in “Exemplary cementformulations”.

In this test (Average temperature: 22.3° C.; Relative Humidity: app.48%) the cement was mixed for 30-60 seconds, then manipulated by handand placed inside the injector. Force was applied via the pistonapproximately 150 seconds after end of mixing, and measurements of forceand piston deflection were taken.

At a time of 2.5 minutes after mixing (0 mm deflection) the forceapplied was higher than 30 N.

At a time of 6.5 minutes after mixing (12 mm deflection) the forceapplied was about 150 N.

At a time of 7.5 minutes after mixing (15 mm deflection) the forceapplied was higher than 200 N.

At a time of 8.5 minutes after mixing (18 mm deflection) the forceapplied was higher than 500 N.

At a time of 9.17 minutes after mixing (20 mm deflection) the forceapplied was higher than 1300 N.

FIG. 4 indicates a viscosity profile of an additional exemplary batch ofcement according to the invention as force (Newtons) vs. displacement(mm). The cement in this test was prepared according to the same formuladescribed for the experiment of FIG. 3. In this test (Average 21.1° C.;Relative Humidity: app. 43%) the cement was mixed for approximately 45seconds, then manipulated by hand and placed inside the injector. Forcewas applied via piston approximately 150 seconds after end of mixing,and measurements of force and piston deflection were taken.

At a time of 2.25 minutes after mixing (0 mm deflection) the forceapplied was higher than 30 N.

At a time of 8.25 minutes after mixing (18 mm deflection) the forceapplied was about 90 N.

At a time of 10.3 minutes after mixing (25 mm deflection) the forceapplied was higher than 150 N.

At a time of 11.4 minutes after mixing (28.5 mm deflection) the forceapplied was higher than 500 N.

At a time of 12.25 minutes after mixing (30 mm deflection) the forceapplied was higher than 800 N.

Results shown in FIGS. 3 and 4 and summarized hereinabove illustratethat exemplary bone cements according to some embodiments the inventionachieve high viscosity in 2.25 minutes or less after mixing iscompleted. Alternatively or additionally, these cements arecharacterized by short mixing time (i.e. transition to highly viscousplastic phase in 30 to 60 seconds). The exemplary cements provide a“working window” for injection of 4.5 to 6.3 minutes, optionally longerif more pressure is applied and/or ambient temperatures are lower. Thesetimes correspond to delivery volumes of 14.9 and 20.8 ml respectively(vertebroplasty of a single vertebra typically requires about 5 ml ofcement). These volumes are sufficient for most vertebral repairprocedures. These results comply with the desired characteristicsdescribed in FIG. 2. Differences between the two experiments may reflectthe influence of temperature and humidity on reaction kinetics.

Molecular Weight Distribution

In an exemplary embodiment of the invention, the average molecularweight (MW) is skewed by the presence of one or more smallsub-population of beads with a molecular weight which is significantlydifferent from a main sub-population of polymer beads. The one or moresmall sub-population of beads may have a MW which is significantlyhigher and/or significantly lower than the average MW.

In an exemplary embodiment of the invention, the presence of even arelatively small sub-population of polymer beads with a MW significantlyabove the average MW causes the cement to achieve a high viscosity in ashort time after wetting of polymer beads with monomer solution.Optionally, increasing a size of the high MW sub-population increasesthe achieved viscosity. Alternatively or additionally, increasing anaverage MW of the high MW sub-population increases the achievedviscosity and/or decreases the time to reach high viscosity.

Optionally, the one or more small sub-population of beads are providedin a formulation in which, the average molecular weight of PMMA in allbeads is 80,000, optionally 100,000, optionally 120,000, optionally140,000, optionally 160,000, optionally 180,000, optionally, 250,000,optionally 325,000, optionally 375.000, optionally 400,000, optionally500,000 Dalton or intermediate or lesser or greater values.

In another exemplary embodiment of the invention, the average molecularweight of the acrylic polymer in the beads is in the range of about130,000 to 170,000, optionally about 160,000 Dalton.

In an exemplary embodiment of the invention, a main sub-population ofPMMA beads has a MW of about 150,000 Dalton to about 500,000 Dalton,optionally about 250,000 Dalton to about 300,000 Dalton, optionallyabout 275,000 Dalton to about 280,000 Dalton. Optionally, about 90-98%[w/w], optionally about 93% to 98%, optionally about 95% to 97% of thebeads belong to the main sub-population.

In an exemplary embodiment of the invention, a second high MWsub-population of PMMA beads has a MW of about 600,000 Dalton, to about5,000,000 Dalton, optionally about 3,000,000 Dalton to about 4,000,000Dalton, Optionally about 3,500,000 Dalton to about 3,900,000 Dalton.Optionally, approximately 0.25% to 5% [w/w], optionally about 1% to 4%,optionally about 2% to 3% of the beads belong to this high MWsub-population. Optionally, this high molecular weight sub-populationcomprises a styrene co-polymer. In an exemplary embodiment of theinvention, a higher molecular weight in this sub-population of beadscontributes to a high viscosity within 2, optionally within 1,optionally within 0.5 minutes or less of wetting of polymer beads withmonomer solution.

In an exemplary embodiment of the invention, a third low MWsub-population of PMMA beads has a MW in the range of about 1,000 Daltonto about 75,000 Dalton, optionally about 10,000 Dalton to about 15,000Dalton, optionally about 11,000 Dalton to about 13,000 Dalton.Optionally, approximately 0.5 to 2.0% [w/w], optionally about 1% of thebeads belong to this sub-population.

Optionally the MW sub-populations are distinct from one another. Thiscan cause gaps between sub-populations with respect to one or moreparameters. In an exemplary embodiment of the invention, thesub-populations are represented as distinct peaks in a chromatographicseparation process. Optionally, the peaks are separated by a return tobaseline. Depending upon the sensitivity of detection, a backgroundlevel of noise may be present. Optionally, gaps are measured relative tothe noise level.

Optionally the sub-populations abut one another so that no gaps areapparent. In an exemplary embodiment of the invention, thesub-populations are represented as overlapping peaks in achromatographic separation process. In this case, there is no return tobaseline between the peaks.

Experimental Analysis of an Exemplary Batch of Cement

Sub-populations characterized by an average molecular weight wereidentified and quantitated using chromatographic techniques known in theart. Exemplary results described herein are based upon GPC analysis.Each peak in the GPC analysis is considered a sub-population. Similaranalyses may be conducted using HPLC. Results are summarized in table 1.

Table I: MW distribution of polymer beads based upon GPC analysis of abone cement according to the powdered polymer component described in“Exemplary cement formulations” hereinabove.

TABLE I MW distribution of polymer beads based upon GPC analysis of abone cement according to the powdered polymer component described in“Exemplary cement formulations” hereinabove. Fraction % of total PD1¹Mw² Mn³ 1 96.5 1.957 278,986 142,547 2 2.5 1.048 3,781,414 3,608,941 31.0 1.009 12,357 12,245 100.0 2.955 373,046 126,248 ¹polydispersityindex (PD1), is a measure of the distribution of molecular weights in agiven polymer sample and is equal to MW/Mn. ²MW is the weight averagemolecular weight in Daltons ³Mn is the number average molecular weightin Daltons1 polydispersity index (PDI), is a measure of the distribution ofmolecular weights in a given polymer sample and is equal to MW/Mn.2 MW is the weight average molecular weight in Daltons3 Mn is the number average molecular weight in Daltons

Table I illustrates an exemplary embodiment of the invention with threesub-populations of acrylic polymer beads.

The main sub-population (fraction 1) of PMMA beads has a molecularweight (MW) of 278,986 Dalton. About 96.5% of the beads belong to thissub-population.

A second sub-population (fraction 2) of PMMA beads has MW of 3,781,414Dalton. Approximately 2.5% of the beads belong to this sub-population.

A third sub-population of PMMA beads (fraction 3) has an MW of 12,357Dalton. Approximately 1% of the beads belong to this sub-population.

In an exemplary embodiment of the invention, cement comprising thesethree sub-populations is characterized by a short mixing time and/orachieves a viscosity of 500 to 900 Pascal-second in 0.5 to 3, optionally0.5 to 1.5 minutes from the beginning of mixing and/or which remainsbelow 2000 Pascal-second for at least 6 to 10 minutes after mixing. Ashort mixing time followed by a long working window is consideredadvantageous in orthopedic procedures where operating room availabilityand medical staff are at a premium.

Size Distribution

In an exemplary embodiment of the invention, the bone cement ischaracterized by beads with a size distribution including at least twosub-populations of polymer beads.

In an exemplary embodiment of the invention, polymer bead diameter is inthe range of 10-250 microns, with a mean value of approximately 25, 30,40, 50, 60 microns, or a lower or a higher or an intermediate diameter.In an exemplary embodiment of the invention, sub-populations of beadsare defined by their size.

Optionally, a main sub-population of polymer (e.g. PMMA) beads ischaracterized by a diameter of about 20 to about 150, optionally about25 to about 35, optionally an average of about 30 microns. Beads in thismain sub-population are optionally far smaller than the smallest beadsemployed by Hernandez et al. (2005; as cited above). Presence of smallbeads can contribute to a rapid increase in viscosity after wetting withmonomer.

Optionally a second sub-population of large polymer beads ischaracterized by a diameter of about 150 microns or more. Presence oflarge beads can slow down the polymerization reaction and preventhardening, contributing to a long working window.

Optionally, the remaining beads are characterized by a very smallaverage diameter, for example less than 20, optionally less than 15,optionally about 10 microns or less. Presence of very small beads canfacilitate rapid wetting with monomer liquid during mixing andcontribute to a fast transition to a viscous state with substantially noliquid phase.

Microscopic analysis indicates that the beads are typically spherical orspheroid.

Hernandez et al. (2005; as cited above) examined the possibility ofadjusting the average polymer bead size by combining two types of beadswith average sizes of 118.4μ (Colacry) and 69.7μ (Plexigum) together indifferent ratios. However, Hernandez's goal was a formulation which is“liquid enough to be injected”. All formulations described by Hernandezare characterized by an increase in viscosity from 500 Pascal-sec to2000 Pascal-sec in about two minutes or less (corresponds to window114). Hernandez does not hint or suggest that there is any necessity oradvantage to increasing the size of this window.

Microscopic analysis also indicated that the barium sulfate particlesare present as elongate amorphous masses with a length of approximately1 micron. In some cases aggregates of up to 70 microns in size wereobserved. In some cases, barium sulfate particles and polymer beadsaggregated together. Optionally, aggregates of Barium sulfate andpolymer beads can delay wetting of polymer beads by monomer.

In an exemplary embodiment of the invention, MMA solvates and/orencapsulates the PMMA polymer beads and the viscosity of the initialmixture is high due to the solvation and/or friction between the beads.As the beads dissolve viscosity remains high due to polymerization whichincreases the average polymer MW.

The following table II shows an exemplary particle size distribution,for example, one suitable for the cement of Table I, based on ananalysis of particles within the ranges of 0.375-2000 microns:

TABLE II Particles size distribution of an exemplary powdered componentVol. % 10 25 50 75 90 Max Beads 2.3 25.75 45.07 60.68 76.34 Diameter[microns]

Experimental Analysis of a Second Exemplary Batch of Cement

Another example of a cement kit for mixture includes a liquid and apowder, which includes a mass of acrylic polymer beads. This cement kitis formulated as follows:

(a) liquid (9.2 gr)

-   -   (i) Methylmethacrylate (MMA) 98.5% (vol)    -   (ii) N,N-dimethyl-p-toluidine 1.5% (vol)    -   (iii) Hydroquinone 20 ppm (vol)

(b) powder (20 gr)

-   -   (i) Polymethylmethacrylate (PMMA) 69.39% (weight)    -   (ii) Barium Sulfate 30.07% (weight)    -   (iii) Benzoyl Peroxide 0.54% (weight)

As noted above, in other formulations the amounts may be varied, forexample, to achieve specific mechanical (or other) properties, or theymay be varied and achieve same mechanical properties. In anothervariation, medication may be added to the powder and/or liquid phases.Other liquid phases may be used as well, for example, as known in theart for PMMA-type cements. The ratios may be varied, for example, asdescribed above.

Table III summarizes a molecular weight distribution of the acrylic beadcomponent of this exemplary cement. It is hypothesized that providing anon-normal distribution of molecular weights with a heavier molecularweight component (e.g., by skewing the MW distribution by includingrelatively higher molecular weight beads) provides an increasedimmediate viscosity. In an exemplary embodiment of the invention, thehigher MW beads are in a relatively small amount (for example, less than20%, less than 10%, less than 5%) and have a MW of between 500,000 to2,000,000 Dalton, optionally 600,000 to 1,200,000 Dalton (for example asshown in the table below).

TABLE III MW distribution of polymer beads of a bone cement of thesecond exemplary batch Range of Molecular Weights [Dalton] % of total1,000,000-2,000,000 0.38%   500,000-1,000,000  3.6%   250,000-500,00012.4%   100,000-250,000 36.4%   50,000-100,000 26.6%   25,000-50,00014.2%   10,000-25,000  5.3%    8,000-10,000  0.5%    5,000-8,000  0.4%

In an exemplary embodiment of the invention, the bone cement ischaracterized by beads with a size distribution including at least twosub-populations of different materials. Optionally, at least twosub-populations include polymer (e.g. PMMA) beads and Barium Sulfateparticles. Optionally, the range of particles diameter of the BariumSulfate is 0.01-15 microns, optionally 0.3 to 3 microns, optionally withan average of about 0.5 microns or lesser or intermediate or greatersizes.

In an exemplary embodiment of the invention, polymer bead diameter is inthe range of 10-250 microns, optionally 15-150 microns, with a meanvalue of approximately 25, 30, 40, 50, 60 microns. Lower or a higher orintermediate diameters are possible as well, for example, based on thesetting considerations described above. In some cases, large particlesizes, for example, particles having diameters exceeding 120 microns(e.g., when the average diameter is on the order of 60 microns) are theresult of Barium sulfate primary particle aggregation on PMMA particlebeads.

An exemplary distribution of bead sizes for the exemplary cement oftable III, based on an analysis of particles within the range of0.04-2000 microns, is described in Table IV:

TABLE IV Particles size distribution of a second exemplary powderedcomponent of bone cement Vol. % 10 25 50 75 90 Max Beads 2 9 46.5 70.790.5 Diameter [microns]

FIG. 5 is a graph which visually shows the values of table IV

Size and MW are Independent Variables

In an exemplary embodiment of the invention, size based and MW basedsub-populations are determined independently. For example, MW may bedetermined chromatographically and size may be determined by microscopicanalysis. As a result, beads classed in a single size sub-population maybe classed in two or more MW sub-populations and/or beads classed in asingle MW sub-population may be classed in two or more sizesub-populations.

Mechanical Viscosity Increasing Agents

In an exemplary embodiment of the invention, the cement includesparticles characterized by a large surface which do not participate inthe polymerization reaction. The large surface area particles can impartadded viscosity to the cement mixture independent of polymerization.Optionally, the added viscosity comes from friction of particles againstone another in the cement.

Examples of materials which do not participate in the polymerizationreaction but increase viscosity include, but are not limited toZirconium, hardened acrylic polymer, barium sulfate and bone.

Optionally, materials which do not participate in the polymerizationreaction but increase viscosity can at least partially substitute forhigh MW polymers in influencing a viscosity profile.

Desired Polymerization Reaction Kinetics

In an exemplary embodiment of the invention, mixture of polymer andmonomer produces a high viscosity mixture with substantially nointervening liquid phase within 180, optionally within 120, optionallywithin 100, optionally within 60, optionally within 30, optionallywithin 15 seconds or greater or intermediate times from onset of mixing.

In an exemplary embodiment of the invention, once a high viscosity isachieved, the viscosity remains stable for 5 minutes, optionally 8minutes, optionally 10 minutes or lesser or intermediate or greatertimes. Optionally, stable viscosity indicates a change of 10% or less intwo minutes and/or a change of 20% or less in 8 minutes. The time duringwhich viscosity is stable provides a working window for performance of amedical procedure.

These desired reaction kinetics can be achieved by adjusting one or moreof average polymer MW, polymer MW distribution, polymer to monomer ratioand polymer bead size and/or size distribution.

General Considerations

In an exemplary embodiment of the invention, a powdered polymercomponent and a liquid monomer component are provided as a kit.Optionally, the kit includes instructions for use. Optionally, theinstructions for use specify different proportions of powder and liquidfor different desired polymerization reaction kinetics.

In an exemplary embodiment of the invention, a bone cement kit includingat least two, optionally three or more separately packagedsub-populations of beads and a monomer liquid is provided. Optionally,the kit includes a table which provides formulations based oncombinations of different amounts of bead sub-populations and monomer toachieve desired properties.

It is common practice in formulation of acrylic polymer cements toinclude an initiator (e.g. benzoyl peroxide; BPO) in the powderedpolymer component and/or a chemical activator (e.g. DMPT) into theliquid monomer component. These components can optionally be added toformulations according to exemplary embodiments of the invention withoutdetracting from the desired properties of the cement.

Optionally, an easily oxidized molecule (e.g. hydroquinone) is added tothe liquid component to prevent spontaneous polymerization duringstorage (stabilizer). The hydroquinone can be oxidized during storage.

Optionally, cement may be rendered radio-opaque, for example by adding aradio-opaque material such as barium sulfate and/or zirconium compoundsand/or bone (e.g. chips or powder) to the powder and/or liquidcomponent.

While the above description has focused on the spine, other tissue canbe treated as well, for example, compacted tibia plate and other boneswith compression fractures and for fixation of implants, for example,hip implants or other bone implants that loosened, or duringimplantation. Optionally, for tightening an existing implant, a smallhole is drilled to a location where there is a void in the bone andmaterial is extruded into the void.

It should be noted that while use of the disclosed material as bonecement is described, non-bone tissue may optionally be treated. Forexample, cartilage or soft tissue in need of tightening may be injectedwith a high viscosity polymeric mixture. Optionally, the deliveredmaterial includes an encapsulated pharmaceutical and is used as a matrixto slowly release the pharmaceutical over time. Optionally, this is usedas a means to provide anti-arthritis drugs to a joint, by forming a voidand implanting an eluting material near the joint.

It should be noted that while use of PMMA has been described, a widevariety of materials can be suitable for use in formulating cements withviscosity characteristics as described above. Optionally, other polymerscould be employed by considering polymer molecular weight (averageand/or distribution) and/or bead size as described above. Optionally, atleast some of the beads include styrene. In an exemplary embodiment ofthe invention, styrene is added to MMA beads in a volumetric ratio of5-25%. Optionally, addition of styrene increases creep resistance.

According to various embodiments of the invention, a bone cementaccording to the invention is injected into a bone void as a preventivetherapy and/or as a treatment for a fracture, deformity, deficiency orother abnormality. Optionally, the bone is a vertebral body and/or along bone. In an exemplary embodiment of the invention, the cement isinserted into the medullary canal of a long bone. Optionally, the cementis molded into a rod prior to or during placement into the bone. In anexemplary embodiment of the invention, the rod serves as anintra-medullar nail.

Exemplary Characterization Tools

Molecular weight and polydispersity can be analyzed, for example by Gelpermeation chromatography (GPC) system (e.g. Waters 1515 isocratic HPLCpump with a Waters 2410 refractive-index detector and a Rheodyne(Coatati, Calif.) injection valve with a 20-μL loop (Waters Ma)).Elution of samples with CHCl₃ through a linear Ultrastyragel column(Waters; 500-Å pore size) at a flow rate of 1 ml/min providessatisfactory results.

It will be appreciated that various tradeoffs may be desirable, forexample, between available injection force, viscosity, degree ofresistance and forces that can be withstood (e.g. by bone or injectiontools). In addition, a multiplicity of various features, both of methodand of cement formulation have been described. It should be appreciatedthat different features may be combined in different ways. Inparticular, not all the features shown above in a particular embodimentare necessary in every similar exemplary embodiment of the invention.Further, combinations of the above features are also considered to bewithin the scope of some exemplary embodiments of the invention. Inaddition, some of the features of the invention described herein may beadapted for use with prior art devices, in accordance with otherexemplary embodiments of the invention.

Section headers are provided only to assist in navigating theapplication and should not be construed as necessarily limiting thecontents described in a certain section, to that section. Measurementsare provided to serve only as exemplary measurements for particularcases, the exact measurements applied will vary depending on theapplication. When used in the following claims, the terms “comprises”,“comprising”, “includes”, “including” or the like means “including butnot limited to”.

It will be appreciated by a person skilled in the art that the presentinvention is not limited by what has thus far been described. Rather,the scope of the present invention is limited only by the followingclaims.

1. A bone cement comprising an acrylic polymer mixture, the cementcharacterized in that it achieves a viscosity of at least 500Pascal-second within 180 seconds following initiation of mixing of amonomer component and a polymer component and characterized bysufficient biocompatibility to permit in-vivo use.
 2. A bone cementaccording to claim 1, wherein the viscosity of the mixture remainsbetween 500 and 2000 Pascal-second for a working window of at least 5minutes after the initial period.
 3. A bone cement according to claim 2,wherein the working window is at least 8 minutes long.
 4. A bone cementaccording to claim 1, wherein the mixture includes PMMA.
 5. A bonecement according to claim 1, wherein the mixture includes BariumSulfate.
 6. A bone cement according to claim 4, wherein the PMMA isprovided as a PMMA/styrene copolymer.
 7. A bone cement according toclaim 4, wherein the PMMA is provided as a population of beads dividedinto at least two sub-populations, each sub-population characterized byan average molecular weight.
 8. A bone cement according to claim 7,wherein a largest sub-population of PMMA beads is characterized by an MWof 150,000 Dalton to 300,000 Dalton.
 9. A bone cement according to claim7, wherein a largest sub-population of PMMA beads includes 90-98% (w/w)of the beads.
 10. A bone cement according to claim 7, wherein a highmolecular weight sub-population of PMMA beads is characterized by anaverage MW of at least 3,000,000 Dalton.
 11. A bone cement according toclaim 7, wherein a high molecular weight sub-population of PMMA beadsincludes 2 to 3% (w/w) of the beads.
 12. A bone cement according toclaim 7, wherein a low molecular weight sub-population of PMMA beads ischaracterized by an average MW of less than 15,000 Dalton.
 13. A bonecement according to claim 7, wherein a low molecular weightsub-population of PMMA beads includes 0.75 to 1.5% (W/W) of the beads.14. A bone cement according to claim 4, wherein the PMMA is provided asa population of beads divided into at least two sub-populations, eachsub-population characterized by an average bead diameter.
 15. A bonecement according to claim 14, wherein at least one bead sub-populationof characterized by an average diameter is further divided into at leasttwo sub-sub-populations, each sub-sub-population characterized by anaverage molecular weight.
 16. A bone cement according to claim 14,wherein the PMMA is provided as a population of beads divided into atleast three sub-populations, each sub-population characterized by anaverage bead diameter.
 17. A bone cement according to claim 1, furthercomprising processed bone and/or synthetic bone.
 18. A bone cementaccording to claim 1, characterized in that the cement achieves aviscosity of at least 500 Pascal-second when 100% of a polymer componentis wetted by a monomer component.
 19. A bone cement according to claim1, wherein the viscosity is at least 800 Pascal-second.
 20. A bonecement according to claim 1, wherein the viscosity is at least 1500Pascal-second. 21-35. (canceled)